Fan and clutch control circuit for an air conditioner

ABSTRACT

A fan and clutch control circuit controls the operation and speed of a fan of an air conditioner over a preselected temperature range and controls the operation of a magnetically operated clutch to drive a compressor of the air conditioner at selected temperatures.

United States Patent Till et a]. 1 June 6, 1972 FAN AND CLUTCH CONTROLCIRCUIT [56] References Cited FOR AN AIR CONDITIONER UNITED STATESPATENTS 72 Inventors; James Peter m 341 Blacklatch Lane, 3,455,] 187/1969 Petranek ..62/229 3,545,219 12/1970 Falk ..62/l80 Camp Hill, Pa.17011; William Jeffrey Hudson, Jr., RD. #1, PO. Box 1 l5, Hummelstown,Pa. 17036 Filed: June 8, 1970 Appli No.: 44,493

Primary ExaminerMeyer Perlin Attorney-William J. Keating, Ronald D.Grefe, Gerald K. Kita, Frederick W. Raring, Jay L. Seitchik and John P.Vandenburg A fan and clutch control circuit controls the operation andspeed of a fan of an air conditioner over a preselected temperaturerange and controls the operation of a magnetically operated clutch todrive a compressor of theair conditioner at selected temperatures.

9 Claims, 4 Drawing Figures PANEL CONTROL SWITCH COOL l CLUICH POWERPOWER 35 um n SOURCE s c azrunu FA" 32 1 ms. AIR TEME MOTOR EVAPORATORSENSOR COIL TEMP.

SENSOR CLUTCH 1 IO i v SHEET 10F 3 INVENTORS JAMES PETER TILL momzmw 12353 N mowzww more: mok mol w z: 125 E4 042 ZCDFUE I A $5 6m y T 5:3

PATENTEDJUH s 1972 mohomdmm .EOmEOo WILLIAM JEFFREY HUDSON,JR BY M VMMPAIEIITEUJUH 6 I972 SHEET 3 BF 3 l I I I I i n I EXAMPLE I CONTROL TEMP.BAND! HI SPEED T FAN SPEED RETURN AIR TEMPERATURE IIIII a EVAPORATOR I''R E5. COMPRESSOR 4 I CAR AIR 9 4 OR WATER FAN AND CLUTCH CONTROLCIRCUIT FOR AN AIR CONDITIONER This invention relates to an electricalcontrol circuit and more particularly to a fan and clutch controlcircuit controlling the operating and speed of a fan and the operationof a magnetically operated clutch of an air conditioner.

ln existing air conditioners, fan control is via an electromechanicalswitch which allows only manual selection of high, medium or low fanspeeds. These speeds are fixed and no temperature dependent means toeffect automatic speed regulation of the fan over apresele'ctedtemperature range is provided. The electro-mechanicalcontrol system also does not provide an independent selection of comfortlevel, for example a temperature range of 65-75 F. Moreover, noprovision is made in existing air conditioner control systems toautomatically control the temperature during certain weather con ditionssuch as the comparatively cooler mornings or evenings of hot days.Therefore the fan air that has been driven across the evaporator coilswill be uncomfortably cool unless the fan speed or temperature settingis manually changed.

An object of the invention is to provide a fan and clutch controlcircuit for controlling the operation and speed of a fan of an airconditioner over a preselected temperature range and for alsocontrolling the operation of a magnetically operated clutch to drive acompressor of the air conditioner at selected temperatures.

Another object is the provision of an air conditioner control circuitwherein the fan speed is automatically varied in a linear manner over apreselected temperature range.

A further object is to provide an air conditioner control circuitpermitting independent selection of the comfort level in the airconditioner area.

An additional object is the provision of an air conditioner controlcircuit that automatically disengages the clutch when temperaturebecomes too cold or drops a prescribed amount below the low side of fantemperature range thereby allowing the air-conditioned area to warm up;however, when air temperature raises to a prescribed amount above thelow side of the fan temperature range, the clutch is automaticallyengaged to place the air conditioner under normal operation.

Other objects and attainments of the present invention will becomeapparent to those skilled in the art upon a reading of the followingdetailed description when taken in conjunction with the drawings inwhich there is shown and described an illustrative embodiment of theinvention; it is to be understood,

Contact 6 is connected via lead 9 to one side of magnetic clutch 10which has the other side connected to ground. Lead 11 connects contact 7to one side of fan motor 12 which has the other side connected toground. Lead 13 is connected to contact 8 and provides a common sourceof positive voltage for control circuitry. Lead 14 connects speedcontrol amplifier 15 to lead 13 while lead 16 connects one side ofcomfort selector 17 through a resistor 39 to lead 13, the other side ofcomfort selector 17 being connected to speed control amplifier 15 vialead 18. Return air temperature sensor 20 is connected to speed controlamplifier 15 via lead 19. I

The output of speed control amplifier 15 is connected via lead 21 to fanpower amplifier 22 and lead 23 connects fan power amplifier 22 to-lead13. The output of fan power amplifier 22 is connected via lead 24 to'theone side of fan motor 12 and also to the input of clutch overrideamplifier 25. The output of clutch override amplifier 25 is connectedvia lead 26 to clutch control amplifier 27. Lead 29 connects lead 13 toone side of DC regulator 28 while lead 30 connects the other side of theDC regulator to the input of clutch override amplifier 25, clutchcontrol amplifier 27, and speed control amplifier 15. Lead 290 connectslead 13 to the clutch override amplifier 25. Lead 31 connects clutchcontrol amplifier 27 to lead 13 while lead 32 connects evaporator coiltemperature sensor 33 to clutch control amplifier 27. Lead 34 connectsthe output of clutch control amplifier 27 to clutch switch 35 which hasone side connected to lead 13 via lead 36 while lead 37 connects theother side to magnetic clutch 10.

Turning now to FIG. 2, comfort selector 17 comprises a rheostat having astationary side 38 connected to lead 13 via lead 1b, including aresistor 39, while movable side 40 is connected to lead 18. Speedcontrol amplifier 15 includes a transistor 41 having the emitterconnected to lead 14, the base connected to lead 18 via resistor 41::and the collector connected to lead 21. A resistor 42 isconnectedbetween the base and collector of transistor 41. Retum air temperaturesensor 20 is a thermistor 43 having one side connected to lead 19 whilethe other side thereof is connected to one side of a resistor 44 in DCregulator 28 via lead 43a, the other side of resistor 44 being connectedto ground. The one side of resistor 44 in DC regulator-28 is connectedto one side of a zener however, that this embodiment is not intended tobe ex haustive nor limiting of the invention but is given for purposesof illustration in order that others skilled in the art may fullyunderstand the invention and the principlesthereof and the manner ofapplying it in practical use so that they may modify it in variousforms, each as may be best suited to the conditions of a particular use.

In the drawings:

FIG. 1 is a block diagram of a fan and clutch control circuit for an airconditioner;

FIG. 2 is a wiring diagram of the circuit of FIG. 1;

FIG. 3 is a curve showing the relationship between the return airtemperature of the air conditioner and its fan speed; and

FIG. 4 is a schematic diagram of the conventional air conditionerrefrigeration system for a vehicle, showing the association of sensor ofthe invention control circuit therewith.

Turning to the drawings and more particularly FIG. 1, a block diagram ofthe fan and clutch control circuit for an air conditioner is illustratedand it includes a source of supply 1 such as, for example, a l2-voltbattery having a negative side grounded with the positive side connectedto common movable contacts 2, 3 and 4 of a manually operated selectorswitch 5. Movable contacts 2 and 3 are disposed opposite a pair ofstationary contacts 6 and 7 while movable contact 4 is disposed oppositeanother stationary contact 8. In one position of switch 5, movablecontact 4 is electrically engaged with stationary contact 8 while in theother position, movable contacts 2 and 3 are in electrical engagementwith stationary contacts 6 and 7, respectively.

diode 45 while the other side thereof is connected to lead 29 Fan poweramplifier 22 comprises transistor 46 having the base thereof connectedto lead 21 and to the junction of resistors 42 and 47, the collectorbeing connected to the base of transistor 48 and the emitter beingconnected to the collector of transistor 48. The emitter of transistor48 is connected to lead 23 while the collector of transistor 48 isconnected to resistor 47 and lead 24.

- Clutch override amplifier 25 includes serially connected resisters 49and 50 connected between lead 24 and lead 13 with the base of transistor51 connected to the junction of these resistors. The emitter oftransistor 51 is connected to lead 13 via resistor 52 and to the emitterof transistor 53 while the collector of transistor 51 is connected tolead 430 via resistor 54 and to the base of transistor 53 by means ofresistor 55. The collector of transistor 53 is connected to the base oftransistor 56 via resistor 57. The collector of transistor 56 isconnected to lead 13 via resistor 58 and the emitter is connecteddirectly to lead 43a.

Clutch control amplifier 27 includes resistors 59 and 60 connectedbetween lead 13 and lead 43a with the junction thereof connected to thebase of transistor 61 via resistor 62. The collectors of transistors 61and 63 are connected together and they are connected to series connectedresistors 64 and 65. The collectors of transistors 66 and 67 areconnected together and to resistor 65. The junction of resistors 64 and65 is connected to lead 31. The emitters of transistors 61, 63, 66 and67 are connected together and the junction therebetween is connected viaresistor 68 to lead 430. The base of transistor 63 is connected to lead26 via resistor 69. The base of transistor 66 is connected via resistor70 to resistor 68. The base of transistor 67 is connected to lead 430via serially connected resistors 71, 71a. Resistor 72 is connectedbetween the junction between resistor 64 and the connection between thecollectors of transistors 61 and 63 and lead 32 which is connected toone side of thermistor 73 defining the evaporator coil temperaturesensor 33. The other side of thermistor 73 is connected to lead 13.

Clutch switch 35 comprises transistor 74 which has the base thereofconnected to lead 34 while the emitter is connected to the base oftransistor 75. The collectors of transistors 74 and 75 are connectedtogether and to lead 37. The emitter of transistor 75 is connected tolead 36.

Referring to FIG. 4, a conventional air conditioner refrigeration systemwith which the above-described control circuits are used is shown asincluding an evaporator 150 arranged in fluid communication with theconditioned space, air being forced in circulation by the fan 152 (whichis driven by motor 12, of FIG. 2). The system further comprises acondenser 154 connected to the evaporator, a compressor 156, and anexpansion valve 158 connected in the line from the compressor back toevaporator 150. The return air temperature sensor 20 (of FIGS. 1 and'2)is mounted in any suitable position in the region of air return to theair conditioner, such as in the return air duct to the fan. Theevaporator coil temperature sensor 33, however, must be mounted directlyon the coil of evaporator 150, as shown in FIG. 4.

With front panel selector switch 5 in MAX COOL position, positive DCsource power is diverted around control circuitry and fed directly tofan motor 12 and magnetic clutch giving the user the option of maximumcooling capacity and this constitutes by-pass circuit means.

DC regulator 28 comprising zener diode 45 across control circuitry andseries dropping resistor 44 to ground provides source of regulated DCvoltage to all control circuits when source voltage exceeds breakdownvoltage of zener 45 and assures stability of operation during nominalfluctuations of source voltage.

With switch 5 in AUTO position, positive DC source power is applied toDC regulator 28, fan speed power amplifier 22, clutch power switch 35and their control amplifiers l5 and 27 respectively.

Fan speed control consists of comfort selector rheostat 17 mounted tothe front panel, return air sensor 20, speed control amplifier 15 andfan power amplifier 22.

With selector switch 5 in the AUTO" position, comfort selector rheostat17 set at some mid-range position allowing, for example, a 70 to 80 Fcontrol temperature band, and initially return air being at sometemperature above 80 F, typical operation is as follows:

Referring to FIG. 2, resistor 39 and pre set comfort selector rheostat17 determine the fixed upper arm of a biasing network whereas thermistorsensor 43 positioned in return air path of fan is the variable lower armof this network which controls the base-emitter bias voltage for speedcontrol transistor 41, that in turn controls base current supplied tocomplementary Darlington connected transistors 46 and 48 of series fanpower amplifier 22.

Initially when return air temperature is above 80 F, at point 101 ofFIG. 3, high speed operation will result, where thermistor 43 resistanceis lowered and bias network provides maximum base voltage to transistor41. This causes transistor 41 to saturate and produce low outputimpedance. The resulting low series impedance of 41 allows maximum basecurrent to flow into the base of transistor 46, produces maximumcollector current thru 46 and in turn maximum base current flow out ofpower transistor 48. As a result, transistor 48 becomes saturated withminimum collector to emitter voltage drop. This allows maximum sourcevoltage to be delivered to the fan load with resulting maximum fancurrent driving fan motor at high speed.

As the fan provides high speed air across the cooling coils of theevaporator, the outlet air will begin to cool the area to beconditioned. This cooler air will eventually return to the fan throughthe return air duct, pass across return air sensor 20, and coolthermistor 43. The resistance of thermistor 43 will now increase andaffect a reduction in base-bias voltage to transistor 41, thus bringingtransistor 41 out of saturation and increasing its output impedance.Thisincreased series impedance presented to the base of transistor 46will effectively reduce its base current and increase its outputimpedance. As a result, the base current flowing out of power transistor48 will proportionally reduce, pulling it out of saturation and affect aproportional increase in its emitter to collector output impedance andseries voltage drop. This will reduce voltage.

available to the fan motor, decrease its load current and in turn lowerfan motor speed thereby placing it at some point on linear portion 102of the fan speed curve, FIG. 3. The reduced velocity of air flowingacross the cooling coils will in turn deliver reduced cooling capacityto the area to be conditioned. This action will now tend to graduallyreduce the rate of change of the resistance of thermistor 43 and slowdown cooling until either temperature stabilization between return airsensor and area to be conditioned is reached within desired temperaturesetting of comfort selector rheostat, or until low speed point 103 ofFIG. 3 is reached as determined by the limiting value of resistor 42which effectively shunts the now high output impedance of transistor 41.Any further decrease in return air temperature will only produce aslight reduction in fan speed as illustrated by 104 of FIG. 3. Clutchcontrol consists of clutch control amplifier 27, evaporator coiltemperature sensor 33 and clutch power switch 35.

Clutch control amplifier transistors 61, 63, 66 and 67 are connected ina differential amplifier configuration with feedback resistor 72 forminga regenerative bistable circuit used as a DC level detector thatprovides digital ON-OFF signals to clutch power switch 35 in response tolinear signals developed across a resistive bridge network consisting offixed resistors 59, 60, 71a, and variable resistance of thermistor 73..Therrnistor 73, which acts as the evaporator coil temperature sensor, isbonded into the evaporator coil and its resistance varies proportionallywith the coil temperature it senses.

During normal clutch operation, transistors 61 and 67 are the onlyactive elements involved. Unused transistor 66 is rendered inoperativeor reverse biased by connecting its base via resistor 70 to its emitterjunction. Transistor 63 is also reverse biased during normaloperation'by clamping its base to lead 43a, via normally saturatedclutch override output transistor 56.

At elevated compressor coil temperatures, the resistance of thermistor73 will decrease causing base bias of transistor 67 to be higher than afixed bias voltage supplied to transistor 61 by resistors 59 and 60. Inthis condition, transistor 67 will be conducting or switched on andtransistor 61 will be non-conducting or off. When transistor 67 turnson, it will draw collector current through resistor 65, thus developingsufi'icient bias voltage at the base of transistor 74 to switch on bothtransistors 74 and 75. As a result, source power is applied to theclutch coil thereby energizing the clutch and engaging the compressor.

The base bias requirements for ON to OFF" state of transistor 67 aredetermined by a voltage divider network consisting of resistors 64, 72,thermistor 73, and resistor 71a. Feedback resistor 72 will effectivelyshunt thermistor 73 resistance since with transistor 61 off it willessentially be tied to positive source voltage at lead 13 via lowresistance of resistor 64. The low parallel resistance of resistor 72and thermistor 73 will raise bias voltage and supply a large basecurrent to transistor 67.

With the compressor engaged, coolant will flow thru the evaporator coilsand cool fan air passing across them. As tem perature of the coilsdecreases, the resistance of thermistor 73, now in parallel withresistor 72, will increase and gradually reduce the bias on the base oftransistor67 until a level is reached whereupon its base voltage isslightly below that of transistor 61 base. Transistor 61 will now turnon, conduct current through resistor 64 and drop the voltage level atthe junction of resistors 64 and 72 near or below that of the base oftransistor 67. This effectively takes feedback resistor 72 out of thebias circuit, further reducing base current available to transistor 67thus regeneratively turning it off. With transistor 67 now off,collector current will cease to flow through resistor 65 and remove biassignal to clutch power switch transistors 74 and 75 and in effectturning them off.

This action. will remove source power from the clutch, deenergizing itthereby causing the clutch to be disengaged which stops flow of coolantthrough compressor coils whereby temperature of coils begins to raise.

Due to the conduction of transistor 61, new base bias conditions for OFFto ON state of transistor 67 are established thereby requiringthermistor 73 alone to produce a large reduction in its resistance for awide coil temperature increase in order to allow base of transistor 67to return to its original ON bias state.

This offset action will allow the clutch to cycle compressor on or offover a wide temperature differential of F or more, thus compensating forthermal time lag of air conditioning system.

Clutch override provides marginal weather control where indirect sensingof return air temperature is performed by monitoring voltage drop acrossfan power amplifier 22 by means of voltage divider resistors 49 and 50.

During normal operation of the fan and clutch, the override circuit willbe deactivated since voltage drop across power amplifier 22 will beinsufficient to bias transistor 51 on. When transistor 51 is off with nocollector current flowing through resistor 54, the junction point ofresistors 54 and 55 will be lowered to almost the level of lead 43athereby effectively biasing transistor 53 on. The collector current ofturned on transistor 53 flowing through resistor 57 will bias transistor56 on and into saturation. This will effectively clamp the base oftransistor 63 in clutch control amplifier 27 to lead 43a, thus reversebiasing transistor 63 and maintain it in an off state.

When return air temperature and fan speed drop to some predeterminedtemperature level 105, FIG. 3, below low speed point 103, the voltagedeveloped across fan power amplifier 22 will raise the voltage at thejunction of resistors 54 and 55 and regeneratively turn transistor 53off, thus stopping flow of base current into transistor 56, turning itoff and stopping flow of collector current through resistor 58. Thiseffectively raises the junction point of resistors 58 and 69 to almostthe level of lead 13, increases bias to the base of transistor 63 andgates it on.

If the clutch happened to be energized at this time, transistor 67 wouldbe conducting; however, the gating action of transistor 63, in responseto fan voltage being monitored will effect turning the clutch off, thusoverriding any coil sensor signals and inhibiting normal clutch actionuntil another predetermined temperature level 106 is reached above thelow speed point 103 of FIG. 3 whereupon the override circuit will revertback to its original state and allow the clutch to resume its normaloperation.

This override loop between the fan and clutch circuits is advantageousfor marginal weather conditions where desired temperature is equal to orslightly below outside ambient temperature and the area to beconditioned does not necessarily need a large amount of cooling but onlya slight amount of cooled air.

It will, therefore, be appreciated that the aforementioned and otherdesirable objects have been achieved; however, it should be emphasizedthat the particular embodiment of the invention, which is shown anddescribed herein, is intended as merely illustrative and not asrestrictive of the invention.

What is claimed is:

1. An electronic circuit for controlling the operation and speed of afan of an air conditioner over a preselected temperature range and theoperation of a compressor of the air conditioner at selectedtemperatures, said circuit comprising speed control circuit means havinga return air temperature sensor means connected thereto to supply acontrol signal to said speed control circuit means corresponding to theair being sensed in a return air section of the air conditioner,

power amplifier circuit means connected to said speed control circuitmeans to receive an output signal from said speed control circuit meansand provide an amplified output signal, a fan motor connected to saidpower amplifier circuit means and adapted to be operated thereby at aspeed in accordance with the amplified output signal, clutch controlcircuit means having an evaporator coil temperature sensor meansconnected thereto to supply another control signal to said clutchcontrol circuit means corresponding to the temperature of the evaporatorcoil, switch means connected to said clutch control circuit means toreceive an output signal from said clutch control circuit means andoperate said switch means, an electrically operated clutch connected tosaid switch means for operation thereby to operate the air conditionercompressor when said switch means is operated, and override circuitmeans connected between said power amplifier circuit means and saidclutch control circuit means to automatically deenergize saidelectrically operated clutch when the temperature drops a prescribedamount below a low side of the preselected temperature range. v

2. An electronic circuit according to claim 1 wherein voltage regulatorcircuit means is connected to said override circuit means and saidclutch control circuit means to control the voltage level thereto. 7

3. An electronic circuit according to claim 1 wherein selector circuitmeans is connected to said speed control circuit means to select thepreselected temperature range.

4. An electronic circuit according to claim 1 wherein bypass circuitmeans is connected to said fan motor and said electrically operatedclutch to operate said fan motor and said electrically operated clutchdirectly instead of by said speed control circuit means, power amplifiercircuit means, clutch control circuit means and said switch means.

5. An electronic circuit according to claim 1 wherein said sensor meanscomprise thermistor means.

6. An electronic control circuit for controlling operation of a firstmember over a preselected temperature range and a second member atselected temperatures comprising first control circuit means havingfirst temperature sensing means connected thereto to supply a controlsignal to said first control circuit means corresponding to the ambientcondition of an area adjacent the first member being sensed by saidfirst temperature sensing means, power amplifier circuit means connectedto said first control circuit means to receive an output signal fromsaid first control circuit means and provide an amplified output signal,first load means connected to said power amplifier circuit means andadapted to be operated thereby in accordance with the amplified outputsignal to operate the first member, second control circuit means havinga second temperature sensing means connected thereto to supply anothercontrol signal to said second control circuit means corresponding to theambient condition of another area adjacent the second member beingsensed by said second temperature sensing means, switch means connectedto said second control circuit means to receive an output signal fromsaid second control circuit means and operate said switch means, secondload means connected to said switch means and adapted to be operatedthereby in accordance with the selected temperatures when said switchmeans is operated, and override circuit means connected between saidpower amplifier circuit means and said second control circuit means toautomatically deenergize said second load means when the temperaturedrops a prescribed amount below a low side of the preselectedtemperature range.

7. An electronic control circuit according to claim 6 wherein saidswitch means comprises serially connected transistor means.

8. An electronic control circuit according to claim 6 wherein voltageregulator circuit means is connected to said override circuit means,said first control circuit means, and said second control circuit meansto control the voltage level thereto.

9. An electronic control circuit according to claim 6 wherein by-passcircuit means is connected to said first and second load means tooperate these load means directly instead of by said first and secondcircuit control means, power amplifier circuit means and said switchmeans.

1. An electronic circuit for controlling the operation and speed of afan of an air conditioner over a preselected temperature range and theoperation of a compressor of the air conditioner at selectedtemperatures, said circuit comprising speed control circuit means havinga return air temperature sensor means connected thereto to supply acontrol signal to said speed control circuit means corresponding to theair being sensed in a return air section of the air conditioner, poweramplifier circuit means connected to said speed control circuit means toreceive an output signal from said speed control circuit means andprovide an amplified output signal, a fan motor connected to said poweramplifier circuit means and adapted to be operated thereby at a speed inaccordance with the amplified output signal, clutch control circuitmeans having an evaporator coil temperature sensor means connectedthereto to supply another control signal to said clutch control circuitmeans corresponding to the temperature of the evaporator coil, switchmeans connected to said clutch control circuit means to receive anoutput signal from said clutch control circuit means and operate saidswitch means, an electrically operated clutch connected to said switchmeans for operation thereby to operate the air conditioner compressorwhen said switch means is operated, and override circuit means connectedbetween said power amplifier circuit means and said clutch controlcircuit means to automatically deenergize said electrically operatedclutch when the temperature drops a prescribed amount below a low sideof the preselected temperature range.
 2. An electronic circuit accordingto claim 1 wherein voltage regulator circuit means is connected to saidoverride circuit means and said clutch control circuit means to controlthe voltage level thereto.
 3. An electronic circuit according to claim 1wherein selector circuit means is connected to said speed controlcircuit means to select the preselected temperature range.
 4. Anelectronic circuit according to claim 1 wherein by-pass circuit means isconnected to said fan motor and said electriCally operated clutch tooperate said fan motor and said electrically operated clutch directlyinstead of by said speed control circuit means, power amplifier circuitmeans, clutch control circuit means and said switch means.
 5. Anelectronic circuit according to claim 1 wherein said sensor meanscomprise thermistor means.
 6. An electronic control circuit forcontrolling operation of a first member over a preselected temperaturerange and a second member at selected temperatures comprising firstcontrol circuit means having first temperature sensing means connectedthereto to supply a control signal to said first control circuit meanscorresponding to the ambient condition of an area adjacent the firstmember being sensed by said first temperature sensing means, poweramplifier circuit means connected to said first control circuit means toreceive an output signal from said first control circuit means andprovide an amplified output signal, first load means connected to saidpower amplifier circuit means and adapted to be operated thereby inaccordance with the amplified output signal to operate the first member,second control circuit means having a second temperature sensing meansconnected thereto to supply another control signal to said secondcontrol circuit means corresponding to the ambient condition of anotherarea adjacent the second member being sensed by said second temperaturesensing means, switch means connected to said second control circuitmeans to receive an output signal from said second control circuit meansand operate said switch means, second load means connected to saidswitch means and adapted to be operated thereby in accordance with theselected temperatures when said switch means is operated, and overridecircuit means connected between said power amplifier circuit means andsaid second control circuit means to automatically deenergize saidsecond load means when the temperature drops a prescribed amount below alow side of the preselected temperature range.
 7. An electronic controlcircuit according to claim 6 wherein said switch means comprisesserially connected transistor means.
 8. An electronic control circuitaccording to claim 6 wherein voltage regulator circuit means isconnected to said override circuit means, said first control circuitmeans, and said second control circuit means to control the voltagelevel thereto.
 9. An electronic control circuit according to claim 6wherein by-pass circuit means is connected to said first and second loadmeans to operate these load means directly instead of by said first andsecond circuit control means, power amplifier circuit means and saidswitch means.